The Sun emits a steady stream of potentially deadly cosmic radiation. As long as humans remain within the Earth’s atmosphere, the threat posed by this radiation is practically nil, but any extended trips into deep space require careful shielding to protect astronauts from the threat of radiation sickness or cancer. The exact levels of radiation vary depending on the severity of solar activity, which falls into a number of predictable cycles.

That’s where the problem starts, according to a new study by NASA scientist John Norbury. We already know about the Schwabe cycle, which shows sunspot activity reaches its peak, known as the solar maximum, every 11 years. When this occurs, there’s a big increase in solar flares and coronal mass ejections, which together spread deadly radiation throughout the solar system. The last solar maximum was reached in 2002, so we’re headed for more in 2013, 2024, and 2035. Those last two dates are worrying, considering the current “2025 out of orbit/2035 to Mars” plans of the United States.